Formed glass sheet having controlled edge stresses

ABSTRACT

A cooling ring assembly (20) and method for controlling stresses in a bent glass sheet G produce a strengthened bent glass sheet after the cooling is completed. The cooling ring assembly includes a cooling ring (22) that supports the glass sheet edge (24), an insulator (30) juxtaposed inboard of the cooling ring to reduce the cooling rate, and a cooler (34) for providing increased cooling to at least one localized area (36,40,58) of the glass sheet edge. The cooler (34) is preferably embodied by a pressurized air supply (42).

This is a divisional of application Ser. No. 08/874,831 filed on Jun.13, 1997 now U.S. Pat. No. 5,865,866 as a divisional of application Ser.No. 08/510,591 which was filed on Aug. 2, 1995 and is now U.S. Pat. No.5,679,124.

TECHNICAL FIELD

This invention relates to a cooling ring assembly and a method forcooling a hot formed glass sheet in a manner that controls stresses, andthe invention also relates to the glass sheet that has been cooled onthe cooling ring assembly by the method.

BACKGROUND ART

One way in which laminated glass windows for vehicles have been formedin the past involves placing pairs of glass sheets to be formed ontomolds that are circulated through a hot furnace for gradual heating andgravity sagging on the molds to the formed shape desired. After suchsagging, the glass sheets are slowly allowed to cool to provideannealing that removes most of the stress from the glass sheets. Thisslow cooling is performed along a furnace section having a decreasingtemperature gradient and is referred to as "active" annealing. Edgecompression for such glass sheets is normally about 150 to 200 KG/cm².Normally there is also some separation between the sheets inward fromtheir peripheries due to thermal warpage that is subsequently overcomeby laminating of the sheets to each other by the use of polyvinylbutyral in a conventional laminating process.

Laminated glass windows have also been previously manufactured by pressbending which utilizes either natural convection or low level forced aircooling that advantageously produces a somewhat greater edge compressionof about 300 to 400 KG/cm². This greater edge compression advantageouslymakes the formed glass sheet less susceptible to edge breakage. However,since the integrated sum of the compression and tension of the glasssheet must equal zero, there is a band of tension inboard from the outeredge surface that is necessarily higher for such press formed glasssheets than for glass sheets that are actively annealed so as to havelower edge stresses. More specifically, whereas formed glass sheets thathave been actively annealed normally have a net inner band tension(NIBT) of about 25 to 50 KG/cm² ₁ press formed glass sheets typicallyhave a net inner band tension of about 50 to 100 KG/cm².

Formed glass sheets having a net inner band tension in excess of about60 KG/cm² can result in undesirable breakage which is usually tested forby the "scratch test" for windshields. In the scratch test, either theindividually formed glass sheet or the laminated pair of glass sheetsare abraded using 80 grit aluminum sandpaper in a band about six incheswide immediately inside the glass edge. If cracking occurs withintwenty-four hours of such abrasion, the sample is considered to havefailed the scratch test. This usually occurs with formed glass sheetshaving a net inner band tension in excess of about 60 KG/cm² while thosewith a lesser net inner band tension usually pass this test. Thisscratch test is considered a good measure of the propensity of aninstalled laminated vehicle window to crack from incidental abrasion orfrom stone impact.

It is generally understood that edge compression of less than about 150to 200 KG/cm² can result in breakage of a laminated glass window duringinstallation into a vehicle window opening, such as a front laminatedwindshield. Higher edge compression like the 300 to 400 KG/cm² normallypresent with pressed formed glass sheets reduces the tendency ofbreakage during installation.

U.S. Pat. No. 4,687,501 Reese discloses lightweight bending iron heatshields for glass bending molds operable to shade glass sheets from aheat source. The heat shields are utilized to change the heating rate ofthe different areas of the glass sheets to affect the final shape.

U.S. Pat. No. 5,069,703 d'Iribarne et al discloses a covering fortempering of glass sheets wherein a metallic fabric is used to cover aframe that supports a glass sheet being annealed. This metallic fabricis of low thermal conductivity and reduces the heat sink characteristicsof the support frame.

U.S. Pat. No. 5,285,786 Shetterly et al discloses a glass sheetannealing ring and method for reducing the maximum value of the netinner band tension while maintaining edge compression as the glass sheetis cooled. This is performed by an insulating ring juxtaposed inboard ofthe annealing ring to reduce the cooling rate of the glass sheet inproximity to the area where the net inner band tension is located.However, with such an insulating ring, the cooling rate is not reducedto as great an extent at localized areas where a metallic coating isprovided such as at bus bars of a vehicle window heater or wiper heatersfor a vehicle window.

Another problem with formed glass sheets is that current designs haverelatively abrupt curvatures where breakage can take place prior toinstallation due to the magnitude of forces generated particularlyadjacent the glass sheet edge at such abrupt curvatures. This oftentakes place when a pair of formed glass sheets that are laminated toeach other do not have complementary mating shapes, such that stress isplaced in the glass by forcing each formed glass sheet to the shape ofthe other.

DISCLOSURE OF INVENTION

One object of the present invention is to provide an improved coolingring assembly on which a hot glass sheet that has been formed is cooledin a manner that enhances the mechanical properties of the finallycooled glass sheet.

In carrying out the above object, the cooling ring assembly includes acooling ring that supports the edge of the formed glass sheet for thecooling. An insulator of the cooling ring assembly is juxtaposed inboardof the cooling ring to reduce the cooling rate of the inner edge portionof the glass sheet and thereby reduce the maximum value of the net innerband tension at the inner edge portion after the cooling is completed. Acooler of the cooling ring assembly provides increased cooling to alocalized area of the edge of the formed glass sheet as compared to therest of the edge to increase the surface compression and depth thereofat the localized area after the cooling is completed.

In its preferred construction, the cooling ring assembly has the coolerconstructed as a pressurized air supply that supplies a flow ofpressurized air to the localized area of the formed glass sheet edge forthe increased cooling. This pressurized air supply as disclosed includesa tube having outlets through which the pressurized air flows to thelocalized area of the formed glass sheet edge for the increased cooling.As disclosed, the tube has a rectangular cross section which facilitatesshaping thereof to the required shape while maintaining the properdirection of flow of the pressurized air through its outlets.

As disclosed, the cooling ring assembly has the cooler constructed tocool a plurality of spaced localized areas of the edge of the formedglass sheet. In one construction, the spaced localized areas at whichthe cooling takes place is designed to cool coated locations such asmetallic coatings of the type utilized for bus bars of a vehicle windowheater or one or more wiper heaters for a vehicle window. In anotherconstruction, the localized areas are cooled to provide increased glassstrength at the edge.

In its preferred construction, the cooling ring assembly includes asupport ring, an adjustable mount that adjustably mounts the coolingring on the support ring, and an insulator and cooler support thatmounts the insulator and the cooler on the support ring. The coolingring also preferably includes a metallic mesh that engages the edge ofthe formed glass sheet to provide its support during the cooling whilereducing conductive heat transfer between the cooling ring and thesupported glass surface.

Another object of the present invention is to provide a method forcooling a hot glass sheet that has been formed in a manner that enhancesthe mechanical properties of the glass sheet after the cooling iscompleted.

In carrying out the above object, the method for cooling the hot glasssheet is performed by insulating the edge of the formed glass sheet toretard cooling thereof and reduce the maximum value of the net innerband tension at the inner edge portion after the cooling is completed.This cooling of the formed glass sheet is performed while providingincreased cooling to a localized area of the edge as compared to therest of the edge to increase the surface compression and depth thereofat the localized area after the cooling is completed.

In the preferred practice, the method is performed by providing theincreased cooling of the localized area by pressurized air. The glasssheet cooling method is performed in different ways as disclosed. In onepractice of the method, the increased cooling is performed at alocalized area having a coating that affects cooling, such as a metalliccoating. The increased cooling is disclosed as being performed at aplurality of localized areas that are spaced from each other. In onepractice, the method is performed at spaced localized areas that eachhave a coating such as a metallic coating utilized for bus bars of avehicle window heater or one or more heaters for a vehicle window wiper.In another practice, the increased cooling is performed at spacedlocalized areas of the edge of the formed glass sheet to provideincreased resistance to breakage upon bending about an axis extendingbetween the spaced localized areas.

Another object of the present invention is to provide a glass sheet thathas been formed while hot and subsequently cooled in a manner thatenhances its mechanical properties.

In carrying out the above object, the glass sheet has an edge having anet inner band tension with a maximum value of less than 60 KG/cm². Theedge of the glass sheet also has a localized area that is less than thetotal extent of the edge and that has an increased surface compressionprovided by increased cooling thereof as compared to the rest of theedge so the surface compression of the localized is greater and deeperthan it would be without such increased cooling of the localized area.

In one embodiment, the formed glass sheet has the localized areaprovided with a metallic coating where the increased cooling takes placeto provide the increased surface compression. The formed glass sheet isdisclosed as having a plurality of the localized areas where theincreased surface compression is provided in a spaced relationship fromeach other.

Another embodiment of the formed glass sheet has a pair of the localizedareas with increased surface compression to reduce breakage upon bendingof the formed glass sheet about an axis extending between the pair oflocalized areas which provides a particular advantage adjacent abruptlycurved areas of the formed glass sheet by reducing bending.

The objects, features and advantages of the present invention arereadily apparent from the following detailed description of the bestmodes for carrying out the invention when taken in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a partially broken away plan view of a glass sheet coolingring assembly constructed in accordance with the present invention tocool a formed hot glass sheet shown supported on the cooling ringassembly;

FIG. 2 is an elevational view taken in section along the direction ofline 2--2 in FIG. 1 through the glass sheet cooling ring assembly andthe glass sheet supported thereon;

FIG. 3 is a graphical illustration of stress of a glass sheet edgewherein the horizontal axis represents the distance from the glass sheetedge surface and wherein the vertical axis indicates the net stress ofall compression and tension at each location, with a positive valuebeing net compression and a negative value being net tension;

FIG. 4 is a graphical illustration similar to FIG. 3 but illustratingthe change in the edge stress when the glass sheet is cooled inaccordance with the present invention;

FIG. 5 is a view of a glass sheet edge that is cooled in accordance withthe prior art to have surface compression outside of the phantom lineindicated shape and tension inside of that shape;

FIG. 6 is a view similar to FIG. 5 of a glass sheet edge wherein thecooling has been performed in accordance with the present invention toincrease the depth of the surface compression at the edge of the glasssheet in addition to decreasing the maximum value of the net inner bandtension;

FIG. 7 is a view of another construction of the cooling ring assemblywherein increased cooling is provided at spaced localized areas of theglass sheet edge to increase the surface compression and depth thereofadjacent bends of the glass sheet in order to reduce breakage prior toinstallation;

FIG. 8 is a side view of a glass sheet that is cooled on the coolingring assembly of FIG. 7 and is taken along the direction of line 8--8thereof to illustrate the bends where the localized areas of increasedsurface compression are provided;

FIG. 9 is a view taken in the same direction as FIG. 8 but illustratinga pair of the formed glass sheets laminated to each other to provide alaminated window;

FIG. 10 is a view of the type illustrated in FIGS. 5 and 6 butillustrated the distribution of compression and tension stresses of aglass sheet edge that is cooled by the cooling ring assembly illustratedin FIG. 9 with the increased cooling but without any coating thataffects cooling; and

FIG. 11 is a view similar to FIG. 9 illustrating the manner in which theincreased cooling can be provided adjacent the lower window edge whereair turbulence causes small particles to abrade the glass surface.

BEST MODES FOR CARRYING OUT THE INVENTION

With reference to FIGS. 1 and 2 of the drawings, a cooling ring assemblygenerally designated by 20 is constructed in accordance with the presentinvention to perform the method thereof for cooling a formed hot glasssheet G which is the resultant product that also embodies the invention.The cooling ring assembly 20, the method for cooling the formed hotglass sheet G and the resultant cooled glass sheet G will all bedescribed in an integrated manner to facilitate an understanding of thedifferent aspects of the invention.

The cooling ring assembly 20 supports the glass sheet G after it hasbeen formed but while still hot and permits the glass sheet to be cooledin a manner which controls the resultant stress to enhances itsmechanical strength as is hereinafter more fully described. This coolingring assembly 20 includes a cooling ring 22 that supports the edge 24 ofthe glass sheet G. This glass sheet edge 24 is located just inboard fromthe extreme outer edge surface 25 and includes an outer portion 26 andan inner portion 28. As is hereinafter more fully described, the outeredge portion 26 of the glass sheet edge 24 is completely in compressionwhile the inner edge portion 28 has a net tension that counterbalancesthe compression and forms a band around the periphery of the glass sheetwhich is referred to as net inner band tension. Inboard from the inneredge portion 28, the glass sheet has stresses that are substantiallyunaffected by edge cooling.

With continuing reference to FIG. 2, the cooling ring assembly includesan insulator generally designated as 30 juxtaposed inboard of thecooling ring 22 to reduce the cooling rate of the inner edge portion 28of the glass sheet and thereby reduce the maximum value of the net innerband tension of the inner edge portion after the cooling is completed.This insulator 30 has a surface 32 that opposes the downwardly facingsurface of the glass sheet G inboard from the cooling ring 22 and ismade as a glass fiber matrix material having very low thermalconductivity so as to retard the cooling at the inner edge portion ofthe glass sheet and thereby reduce the net inner band tension of theinner edge portion 28 after the cooling is completed. The specificinsulator material utilized is sold by Thermal Ceramics, Inc. ofAugusta, Ga., United States of America under the Trademark KAOWOOL. Itis also possible to cover the insulator 30 with a stainless steel screento prevent a vacuum applied to an associated bending mold, when cycledwithout a glass sheet, from sucking the glass fibers from the matrixmaterial.

With combined reference to FIGS. 3 and 5, it will be noted that theprior art type of press formed glass sheet has its edge 24 (FIG. 5)inboard from the outer edge portion 26 provided with an inner edgeportion 28 whose surface compression is not only relatively shallow,i.e. close to its oppositely facing surfaces 33, but also has as shownin FIG. 3 a much greater extent of net inner band tension. Morespecifically, FIG. 5 illustrates by a phantom line shape the location oftransition between compression and tension stresses where thecompression stress is located outward of the phantom line shape and thetension stress is located within its interior.

As illustrated by FIGS. 4 and 6, slower cooling by use of the insulator30 as described above results in an inner edge portion 28 whose surfacecompression is much deeper from the oppositely facing glass surfaces 33and also has a lower maximum value of the net inner band tension.

With reference back to FIG. 2, the cooling ring assembly 20 alsoincludes a cooler 34 for providing increased cooling to at least onelocalized area of the edge portion 24 of the formed glass sheet ascompared to the rest of the inner edge portion to increase the surfacecompression and depth thereof at the localized area. This cooler 34 asillustrated provides the localized area of cooling at a coating with agreater emissivity than the uncoated glass, such as a metallic coating36. As illustrated in FIG. 1, there may be a plurality of the localizedareas of metallic coatings 36 such as is needed to provide bus bars of avehicle window heater 38 of the electric resistance type and may also beutilized to provide a window wiper heater 40 that prevents freezing onthe wiper during cold weather.

With continuing reference to FIG. 2, the cooler 34 is disclosed asincluding a pressurized air supply 42 that supplies a flow ofpressurized air or any other suitable gas to each localized area of theformed glass sheet edge for the increased cooling. More specifically,the pressurized air supply 42 includes a source 44 of pressurized air oranother suitable gas, a conduit 46 for conveying the pressurized air anda tube 48 having opening outlets 50 through which the pressurized airflows to the localized area of the formed glass sheet edge 24 for thecooling. As specifically illustrated, the tube 48 has a rectangularcross-section that is illustrated as being square. This rectangularcross-section of the tube 48 facilitates its mounting on the coolingring while maintaining the proper direction of flow from the tubeoutlets 50 even when the tube has to be bent in order to conform to theshape of the glass sheet edge involved.

As mentioned above, the cooler 34 is illustrated in FIG. 1 as having aconstruction that cools a plurality of spaced localized areas of theglass sheet edge 24, although in the broadest practice of the inventioncooling of only a single localized area can provide advantageous resultssuch as providing such cooling at a single window wiper heater aspreviously described.

As illustrated in FIG. 2, the support ring assembly 20 includes asupport ring 52 that has an L-shaped cross-section. An adjustable mount54 of the cooling ring assembly 20 is illustrated as being of thethreaded bolt and nut type and adjustably mounts the cooling ring 22 onthe support ring at spaced locations about the ring so as to permitadjustment to the required position around the entire extent of thecooling ring. An insulator and cooler support 56 is also mounted in asuitable manner by the support ring 52 and supports the insulator 30 onwhich the tube 48 of cooler 34 is also shown as being mounted. Also, thecooling ring 22 has a metallic mesh 57 that engages the edge 24 of theformed glass sheet to reduce conductive heat transfer to the coolingring. This metallic mesh is preferably embodied by a stainless steelstrip of hooks of a conventional hook and loop type detachable fastenerand is secured to the cooling ring 22 by spaced welds that are notshown.

The cooler 36 described above increases the cooling rate at eachlocalized area on which there is a metallic coating 36 as illustrated inFIGS. 1 and 2. Provision of the cooler increases the rate of cooling sothat there is a greater extent of compression and depth thereof at theinner edge portion 28 without having a net inner band tension that isexcessively large so as to cause strength problems.

With reference to FIGS. 7 and 8, another embodiment of the cooling ringassembly 20'has the same construction as the previously describedembodiment except as will be noted such that the prior description isapplicable and need not be repeated. With this construction, the cooler34 has at least one localized area 58 where the increased cooling isprovided to increase the mechanical strength thereof by providing agreater compressive stress having a greater depth. More specifically, asillustrated, there are spaced localized areas 58 that are illustratedadjacent glass bends 60 in order to reduce breakage upon bending aboutan axis along these bends between the localized areas that are cooled.The glass sheet actually has two such bends 60 such that there are twosets of the spaced localized areas 58 where cooling takes place. A pairof the formed glass sheets G as illustrated in FIG. 9 can be assembledto provide a laminated window 62 with the use of polyvinyl butyral 64 tosecure the formed glass sheets to each other. With this cooling, thecompression is increased and its depth from the outer edge surface 25 isincreased as illustrated in FIG. 10.

With reference to FIG. 11, another embodiment of the cooling ringassembly 20 has the same construction as the embodiment of FIG. 7 and isutilized to cool a window without any metallic coating but is shown asproviding the localized cooling at a single localized area 58 which isadjacent the lower edge of the window where turbulence takes place andentrains small particles that cause abrasion of the glass in a mannerthat can cause fractures.

The resultant formed glass sheets cooled on the cooling ring assembly20, 20'or 20" have an edge 24 having a net inner band tension with amaximum value less than 60 kg/cm² and also have a localized area withincreased surface compression provided by the increased cooling thereofas compared to the rest of the edge. Thus, the surface compression ofeach localized area is greater and deeper than would be present withoutsuch increased cooling of the localized area. With the glass sheet asillustrated in FIGS. 1 and 2, the localized area has a coating with agreater emissivity than the glass such as the metallic coating 36 aspreviously described where the increased cooling takes place to providethe increased and deeper surface compression. A plurality of thelocalized areas where the increased and deeper surface compression isprovided is a spaced relationship from each other are disclosed by theglass sheets G in FIGS. 1 and 7 while a single such localized area isdisclosed by FIG. 11. The increased surface compression reduces thebending about an axis between such areas such as along each bend 60 asillustrated in FIGS. 8 and 9. When the glass sheet edge 24 does not haveany relatively high emissivity coating at which the increased coolingtakes place, the increased compressive stress is deeper from the edgesurface 25 as opposed to the oppositely facing surfaces 33. Thus, theouter edge portion 26 which is fully compression has a greater dimensionin the FIG. 10 embodiment than in the FIG. 6 embodiment. The tensiondistribution of the FIG. 10 embodiment is thus shifted toward the rightfrom the tension distribution of FIG. 6 embodiment. Both the embodimentsof FIGS. 6 and 7 have the compressive stress deeper from the oppositelyfacing surfaces 33 than when cooling is performed without the insulatoras in the prior art as illustrated by FIG. 5.

While the best modes for carrying out the invention have been describedin detail, those familiar with the art to which this invention relateswill recognize various alternative designs and embodiments forpracticing the invention as defined by the following claims.

What is claimed is:
 1. A glass sheet that has been formed while hot andsubsequently cooled, comprising:an edge having a net inner band tensionwith a maximum value of less than 60 Kg/cm² ; and the edge having alocalized area that is less than the total extent of the edge and thathas an increased surface compression provided by increased coolingthereof as compared to the rest of the edge so the surface compressionof the localized area is greater and deeper than would be presentwithout such increased cooling of the localized area.
 2. A formed glasssheet as in claim 1 wherein the localized area has a coating where theincreased cooling takes place to provide the increased surfacecompression.
 3. A formed glass sheet as in claim 1 or 2 which has aplurality of the localized areas where the increased surface compressionis provided in a spaced relationship from each other.
 4. A formed glasssheet as in claim 1 having a pair of the localized areas with increasedsurface compression to reduce breakage upon bending of the formed glasssheet about an axis extending between the pair of localized areas.
 5. Aglass sheet that has been formed while hot and subsequently cooled,comprising:an edge having a net inner band tension with a maximum valueof less than 60 Kg/cm² ; and the edge having a metallic coating on eachof a plurality of spaced localized areas that are less than the entireextent of the edge and that have increased surface compression providedby increased cooling thereof as compared to the rest of the edge so thesurface compression of the localized area is greater and deeper thanwould be present without such increased cooling of the localized areas.6. A glass sheet that has been formed while hot and subsequently cooled,comprising:an edge having an inner band tension with a maximum value ofless than 60 Kg/cm² ; and the edge having spaced localized areas thatare less than the entire extent of the edge and that have increasedsurface compression provided by increased cooling thereof as compared tothe rest of the edge so the surface compression of the localized areasis greater and deeper than would be present without such increasedcooling so as to reduce breakage upon bending of the formed glass sheetabout an axis extending between the localized areas.
 7. A formed glasssheet as in claim 6 wherein the peripheral edge portion has two sets ofspaced localized areas having increased and deeper surface compressionto reduce breakage upon bending of the formed glass sheet about axesextending between the localized areas of both sets.